shouldnt be a problem but you will need some more bits than engine and box, fitting a 300tdi into a 19j results in the engine being in the wrong place, but you have a new 300 chassis so that fixes that issue and using the correct gearbox helps too, you may need propshafts not sure on that one. you will need a different tunnel as the gear stick is in a slightly different place, the tunnel can be bashed out a bit though. you are better to fit the 300tdi fuel filter housing, 300 exhaust is different from the old but presume you are renewing anyway.
rad is different, make sure you use the 300 rad and intercooler setup which may also need some brackets altered, i have seen one done using the 19j rad and separate oil cooler but looked crap.
the temp gauge will read wrong on the dash i got round this with a new gauge and sender from mgill motorsport on ebay £15 as a 300gauge was much dearer